The present invention relates to a development device that, when forming an image electrographically, changes an electrostatic latent image into a visible toner image using toner inside a development vessel and also relates to a toner replenishment method by which toner that is consumed when forming an image electrographically is replenished in the development vessel of the development device.
In order always to achieve an image having suitable density using electrographic image formation, it is necessary to maintain the amount of toner contained in the development vessel of the development device in a state free from excess and deficiency.
Accordingly, in a development device that uses a single component developer constituted by toner only, toner is replenished into the development vessel so that the amount of toner contained in the development vessel is kept within a predetermined range.
Furthermore, in a development device that uses a two-component developer constituted by toner and a carrier, the concentration of toner in the two-component developer in the development vessel is detected and toner is replenished into the development vessel so that the proportion of toner present in the two-component developer is kept within a predetermined range.
Generally, a replenishment roller in a toner box that is detachably mounted in the development vessel is used to replenish toner into the development vessel. For example, in a development device that uses a two-component developer, whether there is an under toner condition or an over toner condition in the development vessel is detected by whether an output value of a toner concentration sensor such as a permeability sensor positioned in the development vessel is larger or smaller than a predetermined threshold. When an under toner condition occurs in the development vessel, the replenishment roller is rotated and toner is replenished into the development vessel so that the output value of the toner concentration sensor falls equal to or below the threshold.
Also, in a digital image forming apparatus disclosed in JP 2004-126219A, a toner usage amount to be used in image formation is calculated from a number of total pixels in an image and a per-pixel toner consumption amount, and toner is replenished to match the amount of toner to be used.
However, in conventional image forming apparatuses, toner replenishment to the development device is carried out by rotating the replenishment roller at an always constant rotational speed. That is, the toner replenishment amount per unit of time is constant regardless of the density of the image that is to be formed.
Thus, when consecutively carrying out image formation of low density images, there is a tendency for an over toner condition to occur in the development vessel, which causes a problem of fogging in the images. On the other hand, when consecutively carrying out image formation of high density images, there is a tendency for an under toner condition to occur in the development vessel, such that replenished toner is used in the development process without being sufficiently churned in, which causes a problem of white patches occurring in the image due to uncharged toner.
These problems can occur even during image formation of a single image when the density of the image to be formed differs greatly from the average density, such that replenishment of an amount of toner corresponding to the amount of toner to be used cannot be achieved, which incurs a problem of reduced image quality.
An object of the present invention is to provide a development device and a toner replenishment method capable of controlling a toner replenishment amount per unit of time using, as a reference, the amount of toner to be used based on image information of the image to be formed, capable of replenishing an appropriate amount of toner to the development vessel even when carrying out consecutive image formation of a plurality of images having different numbers of pixels and densities, and capable of maintaining good image quality by always keeping toner in an appropriately charged condition.